1. Field of the Invention
This invention relates to a valve stepping drive apparatus for driving a suction/exhaust valve of an engine to open and close the valve by a linear pulse motor disposed on the shaft end portion of the valve.
2. Description of the Prior Art
A conventional apparatus for driving a suction/exhaust valve to open and close the same includes a cam shaft disposed on the upper portion or side face of the engine. A crankshaft, which is the output shaft of the engine, and the cam shaft are connected by rotary transmission means such as a belt, and the cam shaft is driven rotatively in synchronism with the rotational phase of the engine. Owing to this rotative drive, the cam face of the cam shaft pushes the shaft end face of the suction/exhaust valve via a link mechanism such as a rocker arm or pushing rod. Accordingly, the suction/exhaust valve, which is held in the closed state at all times by a spring, is opened by pushing the shaft end face of the valve.
This conventional drive apparatus for opening and closing the suction/exhaust valve results in a large-size engine because the cam shaft and link mechanism must be added to the engine.
Furthermore, since the cam shaft and link mechanism are driven by the output shaft of the engine, some of the engine output is consumed by frictional resistance when the cam shaft and link mechanism are driven. This diminishes the effective output of the engine.
Further, the actuation timing of the suction/exhaust valve cannot be altered during engine operation. Therefore, the valve actuation timing is adjusted in such a manner that a high output will be attained when the engine is running at a prescribed rotational speed. As a result, engine output and efficiency decline when the engine is running at an rpm different from the prescribed rpm.
In order to solve the foregoing problems, an apparatus for driving a suction/exhaust valve by connecting a magnetic body to the shaft end portion of the suction/exhaust valve and attracting the magnetic body by a fixed electromagnet has been disclosed in Japanese Patent Application Laid-Open (KOKAI) Nos. 58-183805 and 61-76713.
However, with the disclosed electromagnet arrangement in the apparatus described in these two publications, the distance between the electromagnet and the magnetic body varies with movement of the suction/exhaust valve, and the electromagnetic force acting between the electromagnet and the magnetic body suddenly changes Consequently, the operation of the suction/exhaust valve becomes highly unstable.